1. Field of the Invention
This invention relates to an electrostatic recording apparatus in which an electrostatic latent image is formed on a recording medium and the latent image is developed to form a visible image, and more particularly relates to an electrostatic recording apparatus provided with a surface potentiometer for detecting the surface potential of the recording medium.
2. Description of the Prior Art
Several methods of controlling the surface potential of the recording medium of an electrostatic recording apparatus to be a constant value have heretofore been proposed. For example, there is a method of controlling the voltage applied to a corona discharging device by the detection output of detector means which detects the surface potential, but according to such method, it has been impossible to correct any variation in surface potential which is caused by a variation in corona current which in turn is caused by a temporary variation in temperature and humidity or a variation in the source voltage of the corona discharging device.
There is also a method which uses a constant current circuit to maintain the corona current value constant, whereas according to this method, it has been impossible to cause the same potential to be produced on a photosensitive medium due to deterioration of the photosensitive medium and variations in other characteristics with time even if the corona current value is constant, and accordingly it has not been possible to maintain the surface potential at a proper value.
Also, in an electrostatic recording apparatus using the conventional surface potentiometer, when the detection output of the detector means which detects the surface potential is held for a long time, it has been impossible to form a stable image because of a variation in the holding voltage with time.
It has also been difficult to bring the potential on a recording medium to an ideal value by one detection output of the surface potential due to variations in charging conditions such as variations in environmental conditions, deterioration of the discharging device, deterioration of the drum, etc. It has therefore been impossible to form a stable image based on the detection of the surface potential.
Also, carrying out the detection of the surface potential by a surface potentiometer and the control by the detection output each time an image is formed leads to a great loss of time. When such detection and control are carried out for each predetermined time, accurate surface potential control has not been possible if temperature or humidity is varied within said predetermined time.
In the conventional electrostatic recording apparatus, there is a method whereby a bias is applied to a developing device as the developing means to control the developing device at a predetermined potential with respect to the latent image potential, whereas if the developing bias is maintained constant, irregularity occurs to the developer having a charge. Also, if the developing bias is constant with respect to a variation in the latent image potential, fog has been created in the background of the image.
Where a detector circuit detects an abnormally low surface potential due to the abnormality of the detector circuit which detects the surface potential of the recording medium, deterioration of the recording medium, damage of the discharging device, etc., the detector circuit tries to increase the output of the high voltage source to thereby enhance the surface potential, but if the output of the high voltage source becomes too great, the corona discharge may change into a glow discharge which may damage the surface of the recording medium or the high voltage source itself.
In the conventional electrostatic recording apparatus, one of the factors which make the recorded image unstable has been the deterioration of the recording medium. If the recording medium is deteriorated, it becomes impossible to accumulate on the recording medium a charge necessary to provide a stable image. However, in the conventional electrostatic recording apparatus, it has been impossible to confirm the deterioration of the recording medium.